The invention relates to formulations and methods of manufacturing spray-dried phenol-formaldehyde resins.
Oriented strand board (OSB) may be manufactured with liquid and powdered phenol-formaldehyde resins. To achieve productivity goals, face and core-layers of the OSB may require resins having different properties. The face-layer resin normally should offer some resistance to premature cure, resistance to sticking to the platen and should have a light color after cure. The core-layer resin should normally be fast curing.
Spray-dried phenol-formaldehyde resins are known. U.S. Pat. No. 4,098,770 described that spray-dried powder resins which were suitable for bonding wood waferboard and which were made by spray-drying resole resin compositions containing non-phenolic polyhydroxy compounds. The resole resins were phenol-formaldehyde and phenol-cresol-formaldehyde resins. The non-phenolic polyhydroxy compounds included glycols, polyhydroxy ethers and polyhydroxy compounds.
U.S. Pat. No. 4,424,300 described a powder resin that was made by spray-drying a liquid resin mixture that contained a novolac resin and a resole resin. The phenol-formaldehyde resins included creosol, xylenol and/or catechol modified resins.
U.S. Pat. No. 4,950,433 described a pre-cure resistant powder resin for wood waferboard/oriented strand board manufacture, which was made by spray-drying a liquid resin composition containing a phenol-formaldehyde resin and a water soluble oxo-boron compound such as sodium borate.
U.S. Pat. No. 5,085,930 described a thermosetting powder resin that was made by spray-drying a composition containing a thermosetting resin and an inert inorganic particle.
Several methods have been suggested for improving curing speed of novolac and resole phenol-formaldehyde resins. U.S. Pat. No. 2,524,079 described a phenol-resorcinol-formaldehyde novolac resin made by heating a sodium hydroxide catalyzed novolac phenol-formaldehyde resin and sodium hydroxide catalyzed novolac resorcinol-formaldehyde resin.
U.S. Pat. No. 2,952,040 described a fast cure felt or woven fibrous compound product, which was made by impregnating the felt or woven fibrous compound with a resole phenol-formaldehyde resin and a sodium hydroxide catalyzed novolac resorcinol-formaldehyde resin.
U.S. Pat. No. 4,251,408 described a resin binder composition desirable for manufacturing abrasives, wherein the resin was a mixture of resole resin and phenol-resorcinol-formaldehyde resin.
U.S. Pat. No. 4,426,484 described a fast curing shell molding compound-resin that contained a solid resole resin and a novolac resorcinol resin. The solid resole resin was made by dehydration of liquid resole resin under vacuum.
Resorcinol has also been suggested to improve the curing speed of novolac resins. U.S. Pat. No. 4,089,839 suggests that mixing 1-10 parts of resorcinol to a novolac resin and a curing agent, can reduce cycle time to produce a shell mold. The curing agent is a formaldehyde donor such as hexamethylenetetramine.
Novolac type resorcinol-formaldehyde resins and phenol-resorcinol-formaldehyde resins are incorporated with a formaldehyde solution, paraformaldehyde or phenol-formaldehyde resin as cold set wood product adhesives. The adhesive systems were described in Houwink, R. and G. Saloman, Adhesion and Adhesives, Vol. 1.2nd Ed., Elselvier Publishing Co. pp. 230-240 (1975) and Dressler, H., Resorcinolxe2x80x94Its Uses and Derivatives, Plenum Press. pp. 85-95 (1994).
U.S. Pat. No. 3,903,041 described a fast cure particleboard adhesive made by mixing a resole phenol-formaldehyde resin with 2-4% of a phenol-resorcinol-formaldehyde resin.
U.S. Pat. No. 5,637,658 described a fast cure resin composition which contains a resorcinol or aminophenol modified resole resin and a formaldehyde donor as the curing agent. The resorcinol or aminophenol modified resole resin was made by further condensing a resole resin with resorcinol or aminophenol. The curing agents can be a formaldehyde based resin, paraforrmaldehyde or hexamethylene tetramine.
U.S. Pat. No. 5,374,678 described an adhesive composition which may be capable of bonding to a hard wood. The adhesive composition was a mixture of phenol-formaldehyde resole, 2.5-5% phenolic novolac resin and 0.25 to 1% of m-amino phenol, p-amino phenol, resorcinol, phloroglucinol or resorcinol-phenol-formaldehyde resin.
What is needed is a stable, spray-dried, fast curing resole resin system. What is further needed is a fast curing spray-dried resole resin composition that contains a highly reactive phenolic compound as a curing accelerator. What is also needed is a fast-curing, spray-dried resole resin that flows adequately under heat and pressure. What is still further needed is a fast-curing, spray-dried, resole resin composition that has a desired storage life at ambient temperature.
An embodiment of the present invention provides a fast curing spray-dried phenol-formaldehyde composition that contains 0.02-0.09 moles of highly reactive phenolic compounds per 100 parts of the solids of the phenolic resin as a curing accelerator. The curing accelerators may include resorcinol, alkyl resorcinols, m-amino phenol and phloroglucinol. The curing accelerator in the powder resin functions as cross-linking agent that reacts with the methylol groups of the phenol-formaldehyde resin under the heat and pressure experienced during manufacturing wood composite products.
The fast curing powder resin composition of the present invention may be made in one embodiment by: (1) preparing a resole liquid phenol-formaldehyde resin that contains the desired methylol groups and molecular weight; (2) eliminating the residual free formaldehyde with a scavenging agent; (3) mixing the prepared resole liquid phenol-formaldehyde resin with a highly reactive phenolic compound; and (4) spray-drying the mixture to prepare a powder resin composition without realizing a chemical reaction between the highly reactive phenolic compound and the phenol-formaldehyde resin. The spray-drying process stabilizes the highly reactive phenolic compound in the phenol-formaldehyde resin composition.
The present invention provides a fast-curing, spray-dried, resole resin that is a powder resin composition that has a desired shelf life for wood composite manufacture. The present invention provides a fast curing spray-dried resole resin composition that contains a highly reactive phenolic compound as a curing accelerator. The present invention also provides a fast-curing, spray-dried resole resin that flows adequately under heat and pressure. The present invention also provides a powder resin composition that can improve wood composite manufacturing productivity at relatively low cost. The present invention still further provides a powder resin composition that tolerates higher variation of wood moisture contents. Therefore, higher quality of the wood composite can be manufactured.
There is provided, according to the principals of the present invention, a method for making a fast cure powder resin composition, and a fast cure powder resin composition made by this method. The method includes the steps of:
1. Preparing a liquid phenol-formaldehyde resin;
2. Reducing the residual formaldehyde-from the liquid resin with a formaldehyde scavenging agent and cooling the resin temperature below 30xc2x0 C.;
3. Mixing a curing accelerator with the cooled liquid resin; and
4. Spray-drying the mixture to form a powder resin that contains less than 5% free water content.
A preferred resole resin is made by a two-stage reaction method that can produce a desired molecular weight distribution. The resin making technique involves two formaldehyde additions and two distinct reaction temperatures. The first stage involves a higher reaction temperature that is favorable to the condensation reaction and produce a higher molecular weight resin. The second stage is a lower reaction temperature that is favorable to the methylolation reaction between formaldehyde and phenol.
For the first stage reaction, the molar ratios of phenol: formaldehyde: sodium hydroxide are preferable 1:(1.2-1.6):(0.15-0.25) and more broadly, 1:(0.9-2.0):(0.1-0.3). temperature is about 90xc2x0 C. to reflux. The resin is reacted at the temperature until the number average molecular weight is preferably 1000 to 1300 and broadly 900 to 1700. The molecular weight was determined by the gel permeation chromatograph (GPC) method.
At the end of the first stage reaction, the temperature is then reduced to 65-70xc2x0 C. for second stage reaction. For the second stage reaction, the second portion of the formaldehyde is added to the resin. Therefore, the final molar ratios of phenol: formaldehyde:sodium hydroxide are preferably 1:(1.8-2.2):(0.15-0.25) and, more broadly, 1:(1.5-2.5):(0.1-0.3) stage reaction preferably is allowed to react at 60-70xc2x0 C. until the free formaldehyde is constant. Usually the reaction time is about 1.5-2.0 hours after making the second formaldehyde addition. The free formaldehyde content can be determined by the well-known hydroxylamine-hydrochloride method. A formaldehyde scavenging agent is then added to reduce the residual formaldehyde in the resin to non-detectable levels.
The liquid resin described in U.S. Pat. No. 4,433,120, incorporated herein by reference, may also be used. Alternatively, the phenol for manufacturing the liquid resin process may be partly substituted with xylenols, cresols, catechol and the naturally occurring alkyl phenols such as cresylic acid. The formaldehyde may be replaced partly or completely with other aldehydes such as para-formaldehyde and acetaldehyde. Sodium hydroxide is preferably a catalyst for the resin manufacture. Alternatively, other alkali metal hydroxides, alkaline earth hydroxides, and metal carbonates such as sodium or potassium carbonate may be used in combination with sodium hydroxide.
In order to achieve the fast curing property of the spray-dried resin, the curing accelerator should not be consumed by reacting with residual formaldehyde in the resin. Therefore, it is essential to reduce the amount of residual formaldehyde with a scavenger before mixing with the curing accelerator. The quantity of the scavenger can be determined by a stoichiometric calculation.
A preferred formaldehyde scavenger is ammonia. The ammonia source can be ammonia gas, aqueous ammonia and/or ammonia salts of organic and inorganic compounds. Aqueous ammonia hydroxide (25-50%) is preferred ammonia source. Alternatively, the formaldehyde scavengers can be selected from ketones, amines and amides.
Moreover, in order to produce a fine-particle size of the spray-dried resin, the resin is preferably formulated with a surfactant at 0.2-0.8% based on the liquid resin weight to reduce the surface tension of the resin.
After completion of the formaldehyde scavenging step, the temperature of the liquid resin is preferably cooled to below 25xc2x0 C.
The fast cure spray-dried resin of the present invention preferably contains about 0.02 mole to about 0.09 mole of the free curing accelerator per 100 parts of the solids phenol-formaldehyde resin. The curing accelerator is homogeneously mixed with liquid resin before spray-drying.
The curing accelerators of the present invention are those phenolic compounds that have a higher reactivity towards formaldehyde and methylol groups of a phenol-formaldehyde resin than does phenol itself. Exemplary curing accelerators of the present invention include dihydroxy phenols, trihydroxy phenols, meta-amino phenols and meta-alkyl phenols.
The dihydroxy phenols are selected from the meta-dihydroxy compounds that include resorcinol and alkyl resorsinols. The alkyl resorcinols that are methyl and higher alkyl group substituted resorcinol are natural products extracted from shale oil or oil sand. Trihydroxy phenol is phloroglucinol. The meta-alkyl phenols include meta cresol and 3,5 xylenol.
The most preferred curing accelerator is resorcinol in view of its high reactivity, relative lower cost and availability.
After spray-drying, the retention of the free curing accelerator in the resin depends on residual formaldehyde content, the temperature of the liquid resin and the time between mixing and spray-drying. It is preferred that the liquid resin is cooled to below 25xc2x0 C. before mixing with the curing accelerator and that spray-drying of the mixture occurs as soon thereafter as possible. Therefore, it is the most desirable that an in-line mixture of the liquid resin and a solution of the curing accelerator be made just prior to beginning the spray-drying process. Using this in-line mixing process, the time between mixing and spray-drying can be less than 5 minutes. In a more preferred embodiment, the in-line mixing process can be performed in the range of from about 0.25 minutes to about 3 minutes.
Alternatively, a batch process may also be used for preparation of the mixture of liquid resin and curing accelerator. Due to longer mixing time prior to spray-drying, lower temperatures are required to reduce the decay rate of the curing accelerator. For example, at 20xc2x0 C., the resin mixture can be spray-dried within 8 hours. The lower the mixing temperature, the longer the shelf life of the mixture.
The powder resin is produced by spray-drying a liquid resin mixture that include a phenol-formaldehyde resin and a curing accelerator. The resin mixture atomized to fine droplets in a hot-air stream and the powder resin composition is separated from the stream of hot air. The powder resin is immediately cooled to below 20xc2x0 C. for bagging and storage.
The curing accelerators are heat sensitive compounds. In order to obtain high retention rate of the free-curing accelerator in the powder resin, the spray dryer is operated at mild conditions. Typically, inlet temperatures are from about 140 to about 160xc2x0 C. and outlet temperatures are from about 70 to about 90xc2x0 C.
For wood strand board application the bonding efficiency of a spray-dried resin is significantly correlated with powder particle size distribution. Normally, the smaller the particle size the more efficient is the powder resin at the bonding. The preferred particle size distribution is on that has about 80-90% of the powder resin having a p article size of less than 75 microns and 60-70% having a particle size of less than 45 microns. It is known in the art of spray-drying that the desired particle size distribution may be obtained by manipulation of spray-drying operation variables including infeed resin solids content and surface tension, speed of the spinning atomizer and the liquid resin feed rate.
A drying agent, preferably calcium silicate, may be added at 0.3-1.0% based on the total weight of the dried resin.
The spray-drying process usually increases the resin molecular weight and decreases the content of the free curing accelerator, because some of the curing accelerator may be consumed by reaction with the methylol group of the phenolic resin. The free curing accelerator that is retained after the spray-drying process should be from about 0.02 to about 0.09 mole per 100 parts of the powder resin. The content of the free curing accelerator can be determined preferably by a high-pressure liquid chromatograph (HPLC) equipped with a C18 column.
Moreover, in order to obtain the desired thermal flow property and high bonding efficiency, the number average molecular weight (Mn) of the powder resin should be 1100-1500 preferably and 1000-1900 broadly.
A sample of liquid resin (6.0 g or alternatively, a sample of powdered resin, 3.0 g) is dissolved in 7:3 v/v N-methylpyrrolidone-water (100 mL), cooled to 3-5xc2x0 C., 1.0 N HCl added until pH 4.00, and hydroxylamine hydrochloride solution added (2 mL; prepared by dissolving hydroxylarnine hydrochloride (70 g) in water (1 L) and adjusting to pH 4.0 with 1.0 N NaOH). After 5 minutes in the ice bath, the solution is autotitrated with 0.1 N NaOH to a pH 4.00 endpoint and volume of NaOH solution recorded (mL).
Calculate the percent free formaldehyde as follows:
(mL NaOH)xc3x97(conc. NaOH)xc3x973.002/sample wt.=% Free Formaldehyde 
A sample of powder resin (0.5 g) is dissolved in acetate buffer and diluted up to 100 mL (buffer prepared by dissolving sodium acetate trihydrate (13.6 g) and acetic acid (5.8 mL) in 1 L of water). A 10 mL portion of the resin solution is treated with 30% sulfuric acid (1-2 drops) and pH confirmed  less than 4. After centrifugation, approx. 3 mL is decanted, filtered through a syringe filter, and a sample (50 xcexcL) analyzed by RP-HPLC (eluant 80% acetate bufferxe2x80x9420% acetonitrile).
Calculate the percent free resorcinol as follows:
Resorcinol conc in solution (xcexcg/mL)xc3x97100 mL/sample wt. (g)xc3x97106(xcexcg/mL)=5% Free Resorcinol
The resorcinol concentration is determined from a standard calibration curve which plots peak height vs. concentration, in which observed resorcinol baseline-corrected peak height is used.